The technical field has been rapidly enhancing to which MEMS (Micro Electro Mechanical Systems) devices using silicon are applied. In recent years, the technique has been applied not only to microturbines and sensors but also to the information and communication fields and the medical field. One of the main technical elements supporting this MEMS technique is anisotropic dry etching of silicon, and developments in the MEMS technique are recognized to be supported by developments in this technical element. The anisotropic dry etching of silicon has technically progressed remarkably for the last few years. However, there are still increasing demands for formation of an aperture of a high aspect ratio. Recently, upon formation of trenches or holes, strict demands have arisen to etched shapes such as the shape of a side wall in addition to the aspect ratio. Particularly in a case of forming a silicon structure with use of a substrate (such as an SOI substrate) provided with an etch stop layer, it is quite difficult to satisfy demands for high verticality as well as a side wall in an excellent shape, in other words, elimination of unordinary shaping of the side wall.
As a conventional measure for forming in such a silicon substrate provided with an etch stop layer a trench or a hole of an aperture having high verticality and a side wall in an excellent shape, there have been disclosed by the applicants of the present application and the like methods of etching by utilizing two frequencies upon application of an electric power to the silicon substrate (Patent Documents 1 and 2). However, the above methods were made for elimination of an unordinary shape referred to as a “notch” in the vicinity of an interface between an etch stop layer and a silicon layer, and consideration has not been sufficiently made to the influence of the change in frequency of the electric power applied to the substrate on the shapes of side walls other than the “notch”.